Several audio problems may occur during voice conferencing. For example, voice conferencing equipment having only mono audio capabilities may have more than one microphone coupled to the equipment's mono input. Because the microphones may be arbitrarily positioned, problems may arise when one of the microphones is “idle”—i.e., not near the participants. If input audio picked up from such an “idle” microphone is used in the mono input during the conference, then the resulting mono output may have undesirable noise or reverberance. To deal with this problem, Polycom's VTX 1000 is a conference phone that can automatically select which microphone is active during the conference so that only one of the phone's microphones is “on” at a time.
Another audio problem encountered in voice conferencing arises when there is a disparity between stereo and mono audio capabilities of the conferencing equipment. For example, endpoints in a multi-way call may have different types of conferencing equipment. Some of the endpoints may have stereo audio capability (left and right audio channels) while others may only have mono audio capability (a single audio channel). For the mono endpoints to transmit stereo audio, the mono audio must be converted to stereo. This mono to stereo conversion can easily be done by duplicating the mono channel in both left and right stereo channels.
On the other hand, for the mono endpoint to receive stereo audio, the stereo must be converted to mono. In the conventional approach of converting stereo to mono, the left and right stereo channels are simply added together to produce a summed mono channel. However, this conversion usually results in quality degradation in voice conferencing applications. For example, the left channel may primarily contain audio of a person talking while the right channel contains echoes of the talker and other noise. In such a situation, converting the stereo to mono by simply adding the left and right channels together will degrade the audio quality because the noise and reverberance from the right channel will have been directly merged with the left channel.
What is needed, therefore, is an approach that can convert stereo to mono without quality degradation during a voice conference.